1. Field of the Invention
This invention relates to multiplexing and demultiplexing equipments in end stations of a bidirectional synchronous digital link in which one of multiplexed signals offers a variable rate at the request of one of the stations and where a resulting signal transmitted on the link has a modulation speed which varies according to the rate requested.
More generally, the invention uses time division multiplexing and demultiplexing techniques (TDM) for digital signals having constant rhythm frequencies or rates which differ from each other in a variable ratio, as well as time base and programmable clock recovery techniques and message signalling between stations such as central monitoring systems and terminals.
2. Description of the Prior Art
The developments in digital local networks have led to the connection of a large number of terminals of different types to an integrated services digital network (ISDN) through a subscriber central monitoring system. In particular, the terminals request totally different rates in the two transmission directions as a function of the tasks to be carried out.
At present, the subscriber monitoring system allocates a specific digital link to each of terminals, the link having a constant rate set once and for all according to the rate used by the terminal. During tasks performed by the terminal, when it is required to transmit or receive a quantity of data which is small in comparison to the total capacity offered by the link, the energy consumption is high in comparison to that strictly necessary for the data transmission. In addition, when a terminal is to be replaced by a more advanced terminal operating with a different rate, or by a terminal of another type, following a new geographical layout of the subscriber terminals, the link interfaces between the central monitoring system and the terminals must be changed to make them compatible with the rates of the new terminals.